Dual-band antennas have many applications. For example, systems in which transmit and receive modes are separated in bandwidth are in use or have been proposed. In systems that feature dual-band operation, it is desirable to provide a single antenna aperture that supports both a transmit and receive modes. In order to operate in different bands through a single aperture, diplexers have been used. In concept, diplexers separate the bandwidth of a wide band radiating structure into two narrower bands. Diplexers typically feature filters that selectively feed low and high frequency radiating elements.
Diplexers can be difficult and expensive to implement. For example, one diplexer design that has been used includes ceramic resonator type filters. However, ceramic resonators are heavy and expensive. In addition, they are more bandwidth limited than other designs. For example, they are difficult or impossible to implement at high frequencies (e.g., greater than C band).
Diplexers have also been developed that feature air loaded, suspended stripline filters. These can provide low insertion loss, wide bandwidth, and high rejection. However, air loaded stripline elements have been difficult and expensive to implement. In a typical air loaded stripline element, a conductive element is suspended within a metallic cavity or shielding structure. Such filters have typically been implemented as discrete components that are interconnected to other circuit components, for example through coaxial connectors. As a result, integration with other circuit components can be difficult. As an alternative, designs have been proposed in which transmission line structures are printed on a substrate that is suspended in a shielding channel. Although providing transmission lines in this way can facilitate integration of the stripline within larger circuits, such designs do not avoid the need to encase or surround the stripline within a metal channel. This has been realized using a bottom metallic piece having a groove on one side of the stripline and a metallic cap on the opposite side of the stripline. Accordingly, the filter is relatively difficult and thus expensive to manufacture, and requires the use of a number of different fabrication technologies.